One of the biggest issues with identifying cause (and many other topics around autism) is that we apply the term "autism" to a wide range of individuals. You have some who are incredibly high-functioning that qualify primarily due to their lack of social ability.

On the other end, you have some who are entirely non-verbal and often non-communicative in any form, entirely unable to care for themselves, and often have accompanying medical issues (gastro, immunity, etc.).

We call both autism, yet they are entirely different other than a few shared characteristics. Is it likely that both groups have the same cause?

I am not a biologist, but from the press (yeah, no anti vaccination sites in Germany!) it is

- Do the parents have autism? - Is the father old? - Is the mother old? - Was the child born (very early)? - Did the mother have certain illnesses during pragnency?

http://www.neurologen-und-psychiater-im-netz.org/kinder-juge...

A lot of the talk lately has been linking older fathers to a rise in autism. The theory being that higher chance of mutations with sperm as you get older (and as you have been introduced to more radiation, carcinogens, etc).

http://www.nytimes.com/2014/02/27/health/mental-illness-risk...

The underlying causes of autism are still not well understood, and are likely multifactorial and complex. Autism is fundamentally a disruption of the brain's neurological processes. This can occur within the neurons themselves, at the neural synapses (connections between neurons), or at a more structural/neural organizational level. As has been pointed out elsewhere, the diversity of autism is very great and this is certainly recapitulated at even the most basic neurobiological levels. Despite the high prevalence of autism, the complexity and diversity of the disorder has made it hard to study autism in labs, in epidemiological settings, and in treatment settings.

Nonetheless, it's possible to break out the causes of autism into two main groups:

1. Genetics. Based on large twin studies and family studies, it is believe that the underlying between 50 and 80% of autism cases are due to a genetic mutation, set of genetic mutations or otherwise influenced by genetics. The advance of sequencing technology in the past five years has made possible to sequence the genes in the human genome across thousands individuals with autism and their families. This has given rise to the idea that "de novo" mutations, new mutations which arise in the child, are a primary cause of autism. Every newborn has a set of new mutations, which are mostly benign or inconsequential. However, a small fraction of new mutations are detrimental to the production of key molecules or proteins needed by neurons to function properly. Researchers have identified that perhaps 20-30% of cases of autism have a new mutation which could plausibly underly autism. However, these mutations are incredible diverse-- the same de novo mutation is seen more than once only very rarely, and the top 5-10 most commonly mutated genes account for only ~1% of autism. The long tail makes it necessary to sequence tens of thousands of individuals to pick up signal. A second set.

2. Environment. It's thought (but poorly understood) that certain environmental factors, especially during pregnancy, could also underly autism. Alcohol, for example, can cause Fetal Alcohol Syndrome which can present with autism-like features. Unfortunately, this half of the equation is not my subject expertise so I'll leave it at that.

As a note about searching for information: While google can be unreliable, Google Scholar (https://scholar.google.com/) can be quite helpful.

Some additional resources:

[1] A review on the de novo genetic hypothesis for ASD that I wrote a few years back: http://www.ncbi.nlm.nih.gov/pubmed/24387789

[2] A large sibling/twin study from California about that heritability of autism: http://www.ncbi.nlm.nih.gov/pubmed/21727249

[3] A recent large-scale sequencing study for autism (sorry, paywall): http://www.ncbi.nlm.nih.gov/pubmed/25363768

[4] A review of ASD epidemiology and etiologies: http://www.ncbi.nlm.nih.gov/pubmed/17367287

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That's a huge problem and imo Google should act. This fuels the anti-vaxxers more than it needs to. Education is a key to fight this problem.

Quite possibly as wide as the spectrum itself, a recent article i ran into talked about brainscans showing idiosyncratic across people on the spectrum, while "normal" people were more uniform.